1. Field of the Invention
This invention relates to a method of shaping a helical gear, and a pinion cutter used therein.
2. Description of the Prior Art
FIG. 1 shows the head portion of a pinion cutter 1 employed in a gear shaper for the formation of a helical gear. The pinion cutter has an angle .phi. of twist which is equal to that on a gear blank G (FIG. 4), but a direction of twist which is opposite to that on the gear blank G. A cutter shaft 2 carrying the pinion cutter 1 is supported on a cutter-spindle 4 which is guided by a helical guide 3. The shaft 2 is adapted to make a spiral reciprocal motion along a spiral groove on the helical guide 3. The spiral groove of the helical guide 3, of which an exploded view is shown in FIG. 3, has a lead L which is determined by the normal module m.sub.n and the angle of twist .phi. specified on the gear blank G, and the number of teeth Z on the pinion cutter 1. The pinion cutter 1 has a normal module m.sub.n and an angle of twist .phi. which are equal to those on the gear blank G. Thus, the spiral lead L is expressed by equation (1 ): ##EQU1## where
L=spiral lead on the helical guide (mm);
m.sub.n =normal module on the pinion cutter (mm);
Z=number of teeth on the pinion cutter; and
.phi.=angle of twist on the pinion cutter (.degree.).
The number Z of teeth on the pinion cutter 1 is one of the factors on which the spiral lead L depends, as is obvious from equation (1). In the event there is a change in the normal module m.sub.n or the angle .phi. of twist on the gear blank, however, a change in the number Z of teeth on the pinion cutter does not always serve to maintain the spiral lead L at an equal value, since the number Z is always a integer. Accordingly, if a gear material having a different normal module m.sub.n or angle .phi. of twist is employed, it has hitherto been necessary to use a different pinion cutter, a different helical guide having a different spiral lead L, and a different slider. The change of the helical guide and the slider is, however, not only a time-consuming job, but also requires a high level of skill for adjustment, resulting in a reduction in operating efficiency. Moreover, it is very uneconomical to keep a lot of different helical guides and sliders, since they are expensive.